Method for reproducing signals recorded on optical recording medium

ABSTRACT

A method for reproducing a magneto-optical recording medium the recording pits of which are erased or relieved with rise in temperature of the recording medium caused by radiation of a readout light beam or an optical recording medium the reflectance of which is changed with rise in temperature of the recording medium caused by radiation of the readout light beam, in which changes in the size of an effective reproducing region due to the temperature of the recording medium may be inhibited. To this end, when the magneto-optical disc 11, for example, is rotated at a constant angular velocity, a detecting unit 17 is provided for detecting the radial position of the magneto-optical disc 11 and the laser power as well as the external magnetic field is controlled depending on an output of the detecting unit 17 for controlling the size of an effective reproducing region to be constant without regard to the linear velocity at each reproducing position.

TECHNICAL FIELD

This invention relates to a method for reproducing signals from anoptical recording medium in which signals are read while a light beam isradiated on the recording medium. More particularly, it relates to amethod for reproducing signals from the recording medium capable ofreproducing the information recorded with high recording density.

BACKGROUND ART

An optical recording medium may be roughly classified into a read-onlymedium, such as a so-called compact disc, and a medium on which signalscan be recorded, such as a magneto-optical disc. With any of theseoptical recording media, it is desired to improve the recording densityto a higher level. It is because a data volume several to more than tentimes that of digital audio signals is required when recording digitalvideo signals and because demands are more for reducing the size of therecording medium, such as a disc and hence the size of a product, suchas a player, even when recording digital audio signals. Besides, alarger recording capacity is desired for data disc in general.

Meanwhile, the recording density of recording the information on therecording medium is governed by the S/N ratio of the playback signals.In typical conventional optical recording and reproduction, the totalarea of a beam spot SP, which is a radiation region of the readout beam,such as the laser beam for the optical recording medium, as shown inFIG. 1A, is a playback signal region. Thus, the reproducible recordingdensity is governed by the diameter D_(SP) of the beam spot of thereadout beam.

If, for example, the diameter D_(SP) of the beam spot SP of the readoutlaser beam is less than a pitch g of a recording pit RP, two recordingpits cannot be present in the spot SP, and the playback output waveformis as shown in FIG. 1B, so that the playback signals can be read.However, if the recording pits SP are formed at a higher density, andthe diameter D_(SP) of the beam spot SP becomes larger than the pitch gof the recording pit RP, as shown in FIG. 1, two or more pits may bepresent simultaneously in the spot SP, so that the playback outputwaveform becomes substantially constant as shown in FIG. 1D. In thiscase, these two recording pits cannot be reproduced separately, so thatreproduction becomes infeasible.

The spot diameter D_(SP) depends on the wavelength λ of the laser lightand on the numerical aperture NA. It is this spot diameter D_(SP) thatgoverns the pit density along the scanning direction of the read-outbeam or the recording track direction, or the so-called line density,and the track density conforming to the track interval betweenneighboring tracks in a direction at right angles to the scanningdirection of the readout light beam, or the so-called track pitch. Theopto-physical limits of the line density and the track density are setby the wavelength λ of the readout beam source and the numericalaperture NA of an objective lens and the read-out limit of 2NA/λ isgenerally accepted as long as the spatial frequency at the time ofsignal reproduction is concerned. For this reason, for achieving highdensity of the optical recording medium, it is necessary to diminish thewavelength λ of the light source of the reproducing optical system, suchas a semiconductor laser as well as to enlarge the numerical aperture NAof the objective lens.

The present Applicant has already proposed an optical recording mediumin which the recordable line recording density as well as the trackdensity may be increased without changing the spot diameter of thereadout beam spot, and a method for reproducing the optical recordingmedium. The optical recording medium capable of reproducing the highdensity information in this manner may be enumerated by amagneto-optical recording medium capable of recording informationsignals and a variable reflectance type optical recording medium at,least capable of reproducing information signals.

The above-mentioned magneto-optical recording medium includes a magneticlayer, such as a rare earth-transition metal alloy thin film, depositedon a major surface of a transparent substrate or light-transmittingsubstrate of e.g. polycarbonate, together with a dielectric layer and asurface protecting layer. The magnetic layer has an easy axis ofmagnetization perpendicular to the film surface and exhibits superiormagneto-optical effect. The laser light is irradiated from the side ofthe transparent substrate for recording/reproducing information signals.Signals are recorded on the magneto-optical recording medium byso-called thermomagnetic recording in which the magnetic layer islocally heated by e.g. laser beam radiation to close to the Curietemperature to reduce the coercivity to zero in this region and arecording magnetic field is applied to this region from outside formagnetization in the direct, ion of the recording magnetic field. Therecorded signals may be reproduced by taking advantage of themagneto-optical effect, such as the so-called magnetic Kerr effect orFaraday effect, in which the plane of polarization of the linearlypolarized light, such as laser beam, is rotated in the direction of themagnetization of the magnetic layer.

The variable reflectance type optical recording medium is produced bydepositing a material changed in reflectance with temperature on atransparent substrate on which phase pits are formed. During signalreproduction, the readout beam is radiated on the recording medium andthe reflectance is partially changed within the scanning spot of thereadout light to read out the phase pits.

In connection with the above-mentioned magneto-optical recording medium,high density reproduction or so-called high resolution reproduction ishereinafter explained.

The present Applicant has previously proposed in e.g. Japanese PatentLaid-Open Publication No. 1-143041 (1989) and Japanese Patent Laid-OpenPublication No. 1-143042 (1989) a method for reproducing informationsignals for a magneto-optical recording medium wherein information bits(magnetic domain) are enlarged, diminished or reduced to zero forimproving the playback resolution. The essential point of the technologyconsists in that the recording magnetic recording layer is anexchange-coupled multilayer film composed of a reproducing layer, anintermediate layer and a recording layer, and in that the magneticdomain heated by the playback laser beam during reproduction isenlarged, diminished or erased at a zone of higher temperatures fordiminishing the inter-bit interference during reproduction to render itpossible to reproduce signals of a period lower than the lightdiffraction threshold. There is also proposed in the applicationdocuments of Japanese Patent Application No. 1-229395 (1989) atechnology in which the recording layer of the magneto-optical recordingmedium is formed by a multilayer film including a magnetically coupledreproducing layer and a recording holding layer, the direction ofmagnetization is aligned in advance to an erased state, the reproducinglayer is heated to a temperature higher than a predetermined temperatureby irradiation of the laser beam and in which magnetic signals writtenon the recording holding layer only in this heated state are read outwhile being transcribed on the reproducing layer to eliminate signalcrosstalk to improve the line recording density and the track density.

The above-described high density reproducing technology may be roughlyclassified into an erasable type and a relief type, shown schematicallyin FIGS. 2A, 2B, 2C, 3A, 3B and 3C, respectively.

Referring first to FIGS. 2A, 2B, 2C, the erasable type high densityreproduction technique is explained. With the erasable type, therecording medium, on which information recording pits RP are exhibitedat room temperature, is heated by irradiation of a laser beam LB toproduce an erased region ER within the beam spot SP of the radiatedlaser beam LB, as shown in FIG. 2B, and the recording pit RP within aremaining region RD within the beam spot SP is read, by way of achievingreproduction with improved line density. In sum, this technique consistsin that, when reading the recorded pit RP within the beam spot SP, theerased region ER is used as a mask to narrow the width d of the read-outregion (playback region) RD to provide for reproduction with anincreased density along the scanning direction of the laser beam (trackdirection), that is the so-called line recording density.

The recording medium for erasable type high density reproduction has anexchange-coupled magnetic multi layer film structure composed of anamorphous rare earth for photomagnetic recording (Gd, Tb)-iron group(Fe, Co) ferrimagnetic film. In an example shown in FIG. 2A, therecording medium has a structure in which a reproducing layer as a firstmagnetic film 61, an interrupting layer (intermediate layer) as a secondmagnetic layer 62 and a recording holding layer as a third magneticlayer 63, deposited in this order on a major surface (the lower surfacein the drawing) of a transparent substrate 60 formed of e.g.polycarbonate. The first magnetic layer (reproducing layer) 61 is e.g. aGdFeCo layer with a Curie temperature T_(C1) >400° C., while the secondmagnetic layer (interrupting layer or an intermediate layer) 62 is e.g.a TbFeCoAl film having a Curie temperature T_(c2) of 120° C. and thethird magnetic layer (recording holding layer) is e.g. a TbFeCo layerwith a Curie temperature T_(c3) of 300° C. Meanwhile, arrow marks in themagnetic films 61 to 63 shown in FIG. 2C represent the direction ofmagnetization of the magnetic domains. H_(read) represents the directionof the reproducing magnetic domain.

The reproducing operation is briefly explained. At an ambienttemperature below a predetermined temperature T_(OP), the layers 63, 62and 61 of the recording medium are magnetically coupled in the state ofstatic magnetic coupling or exchange coupling, while the recordingmagnetic domain of the recording holding layer 63 is transcribed to thereproducing layer 61 by means of the interrupting layer 62. If the laserbeam LB is radiated on the recording medium for raising the mediumtemperature, changes in the medium temperature are produced with a timedelay with the scanning of the laser beam, so that a region at atemperature higher than the predetermined temperature T_(OP), that isthe erased region ER, is shifted slightly towards the rear side of thelaser spot SP in the laser scanning direction. The quantity of thisshifting is related to the scanning speed of the laser light, that is tothe velocity of movement of the recording medium (or linear velocity inthe case of the magneto-optical disc). At the temperature higher thanthe predetermined temperature T_(OP), the magnetic coupling between therecording holding layer 63 and the reproducing layer 61 disappears andthe magnetic domains of the reproducing layer 61 are aligned in thedirection of the reproducing magnetic field H_(read), with the recordingpits being erased on the medium surface. A region RD of the scanningspot SP, excluding a superposed region with the region ER where thetemperature is higher than the predetermined temperature T_(OP),substantially represents a reproducing region. That is, the laser spotSP of the laser beam is partially masked by the region ER where thetemperature becomes higher than the predetermined temperature T_(OP), sothat the small unmasked region becomes the reproducing domain RD toachieve high density reproduction.

Since pits may be reproduced by detecting e.g. the Kerr rotation angleof the beam reflected from a small reproducing region (readout regionRD) in which the scanning spot SP of the laser beam is not masked by themasking region (erased region ER), the beam spot SP is equivalentlyincreased in diameter for increasing the line recording density and thetrack density.

In the relief type high density reproducing technique, shown in FIG. 3B,the recording medium in a state in which information recording pits RPare erased at ambient temperature (initialized state) is irradiated witha laser beam and thereby heated to form a signal detecting region DT, asa recording relieved region, within the beam spot SP of the laser beam,and only the recording pit RP within this signal detecting region DT isread for improving the playback line density.

The recording medium for such high density relief reproduction has amagnetic multilayer structure according to magnetostatic coupling ormagnetic exchange coupling. In an example shown in FIG. 3A, areproducing layer 71 as a first magnetic layer, a reproduction assistantlayer 72 as a second magnetic layer, an intermediate layer 73 as a thirdmagnetic layer 73 and a recording holding layer 74 as a fourth magneticlayer are stacked sequentially on a major surface (the lower surface inFIG. 3) of a transparent substrate 70 formed of e.g. polycarbonate. Thefirst magnetic layer ( reproducing layer) 71 is formed e.g. of GdFeCoand has a Curie, temperature T_(c1) 22 300° C., the second magneticlayer (reproduction assistant layer) 72 is formed e.g. of TbFeCoAl andhas a Curie temperature T_(c2) 120° C., the third magnetic layer(intermediate layer) 73 is formed e.g. of GdFeCo and has a Curietemperature T_(c3) 250° C. and the fourth magnetic layer (recordingholding layer) 74 is formed e.g. of TbFeCo and has a Curie temperatureT_(c4) 250° C. The magnitude of an initializing magnetic field H_(in) isselected to be larger than a magnetic field H_(cp) inverting themagnetization of the reproducing layer (H_(in) >H_(cp)) and sufficientlysmaller than the magnetizing field H_(cr) inverting the magnetization ofthe recording holding layer (H_(in) <<H_(cp)). The arrows in themagnetic layers 71, 72 and 73 in FIG. 3C indicate the direction ofmagnetization in each domain, H_(in) indicates the direction of theinitializing magnetic field and H_(read) the direction of thereproducing magnetic field.

The recording holding layer 74 is a layer holding recording pits withoutbeing affected by the initializing magnetic field H_(in), thereproducing magnetic field H_(read) or the reproducing temperature, andexhibits sufficient coercivity at room temperature and at the playbacktemperature.

The intermediate layer 73 exhibits perpendicular anisotropy less thanthat of the reproduction assistant layer 72 or the recording holdinglayer 74. Therefore, a magnetic wall may exist stably at theintermediate layer 73 when the magnetic wall is built between thereproducing layer 71 and the recording layer 74. For this reason, thereproducing layer 71 and the reproduction assistant layer 72 maintainthe erased state (initialized state) in stability.

The reproduction assistant layer 72 plays the role of increasingcoercivity of the reproducing layer 71 at room temperature, so thatmagnetization of the reproducing layer 71 and the reproduction assistantlayer 72 may exist stably despite the presence of the magnetic wall. Onthe other hand, coercivity is decreased acutely during reproduction inthe vicinity of the reproduction temperature T_(S) so that the magneticwall confined in the intermediate wall 73 is expanded to thereproduction assistant layer 13 to invert the reproducing layer 71ultimately to extinguish the magnetic wall. By this process, pits arecaused to appear in the reproducing layer 71.

The reproducing layer 71 has a low inverting magnetic field H_(cp) sothat the domains of overall surface of the layer 71 may be aligned bythe initializing field H_(in). The aligned domains are supported by thereproduction assistant layer 72 and may thereby be maintained stablyeven if there exist a magnetic field between the layer and thereproduction assistant layer 74. Recording pits are produced by thedisappearance of the magnetic wall between the layer and the recordingholding layer 74 during reproduction, as described above.

If the operation during reproduction is explained briefly, the domainsof the reproducing layer 71 and the reproduction assistant layer 72 arealigned before reproduction in one direction (in an upward direction inFIG. 3C) by the initializing magnetic field H_(in). At this time, amagnetic wall (indicated in FIG. 3C by a transversely directed arrow) ispresent stably so that the reproducing layer 71 and the reproductionassistant layer 72 are stably maintained in the initialized state.

A reproducing magnetic field H_(read) is applied in an inverse directionwhile a laser beam LB is radiated. The reproducing magnetic fieldH_(read) needs to be in excess of the magnetic field which inverts thedomains of the reproducing layer 71 and the reproduction assistant layer72 at a reproduction temperature T_(RP) following temperature increaseby laser irradiation to cause extinction of the magnetic field of theintermediate layer 73. The reproducing magnetic field also needs to beof a such a magnitude as not to invert the direction of magnetization ofthe reproducing layer 71 and the reproduction assistant layer 72.

With scanning by the laser light, temperature changes in the medium areproduced with a delay, so that the region whose temperature exceeds apredetermined reproducing temperature T_(RP) (recording relieved region)is shifted slightly .from the beam spot SP towards the rear side alongthe scanning direction. The quantity of this shifting is related to thescanning speed of the laser beam, that is to the velocity of movement ofthe recording medium (or linear velocity in the case of themagneto-optical disc). With the temperature above the predeterminedreproducing temperature T_(RP), coercivity of the reproduction assistantlayer 72 is lowered, so that, when the reproducing magnetic fieldH_(read) is applied, the magnetic wall is caused to disappear so thatthe information of the recording holding layer 74 is transcribed on thereproducing layer 71. Thus, a region within the beam spot SP which doesnot reach the reproducing temperature T_(RP) is masked and the remainingregion within the beam spot SP becomes the signal detecting region(reproducing region) DT which is the recording relieved region. Highdensity reproduction may be achieved by detecting e.g. the Kerr rotationangle of a plane of polarization of the reflected beam from the signaldetecting region DT.

That is, the region within the beam spot SP of the laser beam LB whichdoes yet not reach the reproduction temperature T_(RP) is a mask regionin which recording pits are not displayed, while the remaining signaldetecting region (reproducing region) DT is smaller in area than thespot diameter, so that the line recording density and the track densitymay be increased in the same manner as described above.

There is also devised a high density reproducing technique consisting ina combination of the erasure type and the relief type. With thistechnique, the laser beam is radiated to the recording medium in aninitialized state thereof in which recording pits are extinct at roomtemperature for heating the recording medium for forming a recordingrelieved region at a site slightly deviated towards the rear side of thebeam spot of the radiating laser beam, while simultaneously forming anerased region of a higher temperature within the recording relievedregion.

In the specification and the drawings of our co-pending JP PatentApplication No. H 2-418110, having the same assignee as the presentapplication (1991), there is proposed a signal reproducing method for amagneto-optical recording medium wherein a magneto-optical recordingmedium having at least a reproducing layer, an intermediate layer and arecording holding layer is employed, a laser beam is radiated and areproducing magnetic field is applied to the reproducing layer, atemperature distribution generated by the laser radiation is utilized toproduce a region where an initialized state is maintained, a region towhich the information of the recording holding layer is transcribed anda region the domains of which are aligned in the direction of thereproducing magnetic field, in a field of view of the lens, to produce astate equivalent to optically masking the field of view of the lens toincrease the line recording density and the track density as well as toassure satisfactory frequency characteristics at the time ofreproduction, there being no risk that, even if the reproducing power isfluctuated, the region of transcription of the information of therecording holding layer be diminished or enlarged.

According to the above-described high density reproducing techniqueemploying such magneto-optical recording medium, only the read regionRD, which is in effect the signal reproducing region, or the recordingpit RP within the signal detecting region DT, is read within the beamspot SP. Since the size of the read region RD or the signal detectionregion DT is smaller than the size of the beam spot SP, the distancebetween adjacent pits in the directions along and at right angles to thelaser light scanning direction may be reduced to raise the line densityand the track density to increase the recording capacity of therecording medium.

Meanwhile, with the above-described method for reproducing thehigh-density information, even although the external reproducingmagnetic field is constant and the laser beam power is constant, thesize of the region RD of FIG. 2B or that of the region DT of FIG. 3B, asthe reproducing region, is fluctuated with the scanning speed of e.g.the laser beam, that is the velocity of movement of the recording medium(or the linear velocity of e.g. a magneto-optical disc).

For example, with the erasure type reproducing method, explained inconnection with FIG. 2B. if the velocity of movement (linear velocity)of the recording medium, such as the magneto-optical disc, is low, thetime necessary for a scanning spot SP to pass through a unit distance ofmovement becomes longer, so that the state of temperature distributiondue to laser beam radiation is such that an erased region at atemperature higher than the Curie temperature T_(c), or a mask region,becomes wider, as shown at a curve a in FIG. 4B and a mask regionER_(LS) for low linear velocity shown in FIG. 4A, so that an effectivereadout region (reproducing region) RD is diminished.

On the contrary, if the velocity of movement (linear velocity) of therecording medium, such as the magneto-optical disc, is high, the timenecessary for the scanning spot SP to pass through a unit distance ofmovement becomes shorter, so that the state of temperature distributiondue to laser beam radiation is such that an erased region at atemperature higher than the Curie temperature T_(c), or a mask region,becomes narrower, as shown at a curve b in FIG. 4B and a mask regionER_(HT) for high linear velocity shown in FIG. 4A, so that an effectivereadout region (reproducing region) RD is enlarged.

With the relief type, as will become apparent from its operatingprinciple, if the temperature of the magneto-optical recording medium ishigh, the reproducing region is increased, whereas, if the temperatureof the magneto-optical recording medium is low, the reproducing regionis diminished.

As described above, if, with the erasure type reproducing method or withthe relief type reproducing method, the velocity of movement of therecording medium is fluctuated, there is a risk that stable reproductionwith a high S/N ratio cannot be achieved. If, when the magneto-opticaldisc is rotated at e.g. a constant angular velocity (CAV) to effectreproduction, the scanning spot position of the reproducing beam, thatis the reproducing position, differs along the disc radius, the readoutregion RD or the signal detecting region DT, as an effective reproducingregion, differs with the reproducing position, because of the differencein the linear velocity, so that stable reproduction with good S/N ratiocan not necessarily be assured.

The same may be said when reproducing a variable reflectance opticalrecording medium by way of high density reproduction or ultra highresolution reproduction. That is, since the portion within the readoutlight beam which is changed in reflectance is changed in size withchanges in the velocity of movement of the recording medium (linearvelocity), the high reflectance portion, which is in effect thereproducing region, is fluctuated in size with the linear velocity ofthe recording medium, so that stable reproduction can occasionally notbe achieved.

SUMMARY OF THE INVENTION

In view of the above-described status of the art, it is an object of thepresent invention to provide a method for reproducing an opticalrecording medium in which, even although the velocity of movement of themagneto-optical recording medium or the variable reflectance typeoptical recording medium is changed, the size of the effectivereproducing region may be maintained constant to assure stable readingof information signals.

According to the present invention, there is provided a method forreproducing information on an optical recording medium comprising arecording layer and a reproducing layer, the recording and reproducinglayers being magnetically coupled to each other in steady state, themethod comprising extinguishing magnetic coupling between the recordinglayer and the reproducing layer in a region the temperature of which israised to a temperature higher than a predetermined temperature byirradiation with a readout laser beam during reproduction, and readingthe recording information held by the recording layer in an area of anirradiated region other than the magnetic coupling extinguished region,characterized by detecting the reproducing position on the opticalrecording medium when rotating the optical recording medium at aconstant rotational velocity for reproducing the recording medium, andcontrolling the size of the second region in accordance with the linearvelocity at the reproducing position.

According to the present invention, there is also provided a method forreproducing information on an optical recording medium having arecording layer and a reproducing layer, the method comprising aligningthe domains of the reproducing layer, transcribing the recordinginformation to the reproducing layer and relieving the transcribedinformation, the recording information being held before transcriptionby a region on the recording layer the temperature of which is increasedby irradiation with a readout beam during reproduction, and reading therecording information from the relived region of the reproducing layer,the method further comprising detecting a reproducing position on theoptical recording medium when rotating the optical recording medium at aconstant rotational velocity for reproduction, and controlling the sizeof the relieved region in accordance with the linear velocity at thereproducing position.

According to the present invention, there is additionally provided amethod for reproducing information on an optical recording mediumcomprising radiating a readout beam to an optical disc on which phasepits are formed in accordance with signals and which is changed inreflectance with temperatures, and reading the phase pits whilepartially changing the reflectance within a scanning spot of the readoutbeam, the method further comprising detecting a reproducing position onthe optical recording medium when rotating the optical recording mediumat a constant rotational velocity for reproduction, and controlling thesize of a portion within the scanning spot of the readout beam in whichthe reflectance is changed.

With the above-described reproducing method for the optical recordingmedium, an output of a laser light source radiating the readout beam onthe optical recording medium may be controlled on the basis of an outputdetecting the reproducing position of the optical recording medium. Theoutput of the laser light source may also be controlled on the basis ofa comparison output between the output detecting the reproducingposition of the magneto-optical recording medium and an output referencevalue of the laser light source which is a value stored in memory meansand associated with each line velocity of the optical recording medium.The size of the magnetic coupling extinguished region, the relievedregion or the region the reflectance of which is changed may also becontrolled depending on the level of an output reproducing the opticalrecording medium.

Thus, according to the signal reproducing method for an opticalrecording medium according to the present invention, since the size ofthe effective reproducing region is not changed even if the linearvelocity of the optical recording medium is changed of the reproducingposition on the optical recording medium and hence the linear velocityat the reproducing position are changed, stable reproduction may beachieved with a high S/N ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C and 1D are diagrams illustrating the relation betweenthe spot diameter of a laser beam and the recording density ofreproducible recording pits.

FIGS. 2A, 2B and 2C are diagrams illustrating an erasable typemagneto-optical recording medium, a method for reproducing the recordingmedium and an effective reproducing region of the recording medium.

FIGS. 3A, 3B and 3C are diagrams illustrating a relief typemagneto-optical recording medium, a method for reproducing the recordingmedium and an effective reproducing region of the recording medium.

FIGS. 4A and 4B are diagrams illustrating that the effective reproducingregion is changed with changes in the temperature of the magneto-opticalrecording medium.

FIG. 5 is a block diagram showing essential parts of a disc reproducingapparatus to which an embodiment of the reproducing method for themagneto-optical recording medium according to the present invention isapplied.

FIG. 6 is a block diagram showing a sector format for data recorded on amagneto-optical disc.

FIG. 7 is a view for illustrating that a mask region is changed bychanging a laser power.

FIG. 8 is a view for illustrating that a mask region is changed bychanging an external magnetic field.

FIG. 9 is a block diagram showing essential parts of a disc reproducingapparatus to which another embodiment of the reproducing method for themagneto-optical recording medium according to the present invention isapplied.

FIG. 10 is a block diagram showing essential parts of a disc reproducingapparatus to which a still another embodiment of the reproducing methodfor the magneto-optical recording medium according to the presentinvention is applied.

FIG. 11 is a view for illustrating a magneto-optical disc of themodified CAV rotating driving system.

FIG. 12 is a block diagram showing essential parts of a disc reproducingapparatus to which another modified embodiment of the reproducing methodfor a magneto-optical disc according to the present invention isapplied.

FIG. 13 is a schematic cross-sectional view showing essential parts ofan example of a phase change type optical disc as typical of a variablereflectance optical disc employed in the embodiment shown in FIG. 12.

FIG. 14 is a schematic cross-sectional view showing another example ofthe phase change type optical disc.

FIG. 15 is a schematic cross-sectional view showing still anotherexample of the phase change type optical disc.

FIG. 16 is a view showing a phase change state for explanation of theabove-mentioned phase change type optical disc.

FIG. 17 is a view showing another phase change state for explanation ofthe above-mentioned phase change type optical disc.

FIGS. 18A and 18B are diagrams showing the relation between thetemperature distribution and a readout light spot for explanation of theabove-mentioned phase change type optical disc.

FIG. 19 is a schematic cross-sectional view showing essential parts ofanother example of the variable reflectance optical disc employed in theembodiment shown in FIG. 9.

FIG. 20 is a graph showing the state of change in reflectance spectralcharacteristics with changes in temperature in an interference filter.

BEST MODE OF CARRYING OUT THE INVENTION

Referring to the drawings, certain embodiments of an optical recordingmedium according to the present invention will be explained. First, anembodiment in which the present invention is applied to amagneto-optical recording medium as a recordable medium, and then anembodiment in which the present invention is applied to a variablereflectance optical recording medium as a recordable medium, will beexplained.

FIG. 5, a magneto-optical recording medium is a magneto-optical disc 11,to which the above-mentioned erasable type or relieved type reproducingmethod is applied. In this case, the magneto-optical disc 11 isrotationally driven in accordance with a constant angular velocity (CAV)system.

For example, the magneto-optical disc to which the erasable typereproducing method is applied has an exchange-coupled magneticmultilayer film structure composed of an amorphous rare earth (Gd,Tb)-iron group (Fe, Co) ferrimagnetic film for magneto-opticalrecording. The multilayer film structure is made up of a recordingholding layer formed e.g. of TbFeCo with a Curie temperature of 300° C.,an interrupting layer (intermediate layer) of e.g. TbFeCoAl with a Curietemperature T of 120° C. and a reproducing layer of e.g. GdFeCo with aCurie temperature of not lower than 400° C. The magneto-optical disc towhich the relief type reproducing method is applied is such a disc inwhich the recording holding layer is formed of e.g. TbFeCo with a Curietemperature of 250° C., the intermediate layer is formed e.g. of GdFeCowith a Curie temperature of 250° C., the reproduction assistant layer isformed e.g, of TbFeCoAl with a Curie temperature of 120° C. and thereproducing layer is formed e.g. of GdFeCo with a Curie temperature ofnot lower than 300° C.

In this case, data are sequentially recorded as plural sectors pertrack. Each sector is arranged as shown for example in FIG. 6. That is,each sector is constituted by a preformating section and a recordingreproducing section. In the preformating section, data are pre-recordedas pits on the magneto-optical disc 11. During recording, thispreformating section is detected and data etc. are recorded only in therecording reproducing section.

The preformating section is made up of a sector synchronizing sectionand an address section. In the address section, address data includingtrack addresses and sector addresses are recorded. The track addressesare serial numbers from e.g. the inner periphery of the disc as therecording start position, or track numbers, corresponding to the radialpositions of the magneto-optical disc 11. The sector addresses indicatethe serial numbers of the sectors in a given track.

In the present embodiment, the radial position of the optical pickup ofthe magneto-optical disc 11, that is the reproducing position, isdetected by detecting the above-mentioned track address, and the laserbeam power is controlled depending on the linear velocity at thereproducing position for maintaining the constant size of thereproducing region (readout region) for the erasable type reproducingmethod or the constant size of the reproducing region (signal detectingregion) DT for the relief type reproducing method.

As a readout beam, a laser beam from a laser source 12, such as asemiconductor laser, is incident on the reproducing layer of themagneto-optical disc 11.

In the present embodiment, a reproducing magnetic field H_(read) isgenerated by the driving current supplied to a magnetic field generatingcoil 31 from a driver 32. The magnetic field generating coil 31 isarranged facing the laser source 11 on the opposite side of themagneto-optical disc 11 with respect to the laser beam side. A referencevalue M_(ref) from a reference value generating circuit 23 is suppliedto the driver 22 and the strength of the reproducing magnetic fieldH_(read) generated by the coil 21 is set so as to be a constant valueconforming to this reference value.

In accordance with the above-mentioned erasable or relief typereproducing method, the reflected light from the reproducing region RDor DT within the beam spot of the laser beam is incident on areproducing photodetector 13 by optical means, not shown, forphotoelectric conversion.

Output signals of the photodetector 13 are supplied via a head amplifier14 to a signal processing circuit 15 to produce RF signals which aresupplied to a data reproducing system for demodulation.

Part of the laser beam from the laser source 12 is incident on a laserpower monitoring photodetector 21. The photoelectrically convertedoutput of the photodetector 21 is supplied to an automatic powercontroller 22, in which the output of the photodetector 21 and areproducing laser power setting reference value REF are compared to eachother. Outputs of the comparison error are supplied to a laser drivingcircuit 23 for controlling the output power from the laser source 12. Bythe above-described close-loop control, the output power of the laserlight source 12 is controlled so as to conform to the reproducing laserpower setting reference value REF.

In the present embodiment, the reproducing laser power setting referencevalue REF is adapted to conform to the linear velocity at each radialreproducing position of the magneto-optical disc 11, as will now beexplained.

That is, there is provided a ROM 24 which stores a table of reproducinglaser power setting reference values REF, associated in a one-for-onerelationship to the linear velocities at the respective track positionsof the magneto-optical disc 11. Such reproducing laser power settingreference values REF, which will give constant sizes of theabove-mentioned effective reproducing region (readout region RD orsignal detecting region DT) suitable for reproduction when the erasabletype magneto-optical disc or the relief type magneto-optical disc isreproduced at the respective linear velocities at the respectivereproducing track positions, are previously detected and stored in ROM24.

Whether or not the size of the reproducing region RD or DT is of anoptimum constant value may be detected depending on whether or not theRF signal level from the signal processing circuit 15 is of apredetermined value when the predetermined reference pattern informationis reproduced.

In an address decoder 17 of a data reproducing system 16, trackaddresses are extracted from reproduced signals and discriminated. Thesetrack addresses are supplied to ROM 24 as readout addresses for ROM 24.Different values of the laser power setting reference values REF areread out depending on the linear velocities at the reproducing trackpositions. The read-out setting reference values REF are supplied toautomatic power controller 22 whereby the output power of the laserlight source 12 is controlled to conform to the setting reference valuesdepending on the linear velocities at the reproducing positions on themagneto-optical disc 11.

As described above, if the radial reproducing positions on themagneto-optical disc 11 are changed, the temperature distributionrelated to the laser beam scanning spot is changed depending on thelinear velocities of the disc at the reproducing position. If the laseroutput power is changed for the constant linear velocity of themagneto-optical disc 11, the size of the region at a temperature inexcess of the predetermined threshold temperature TΘ is changed as shownat S1, S2 in FIG. 7. Thus, by controlling the laser power as describedabove, the constant size of the above-mentioned reproducing region RD orDT may be maintained despite changes in the linear velocity of themagneto-optical disc 11.

In this manner, even although the radial reproducing position of themagneto-optical disc 11 and hence the linear velocity is changed, theconstant size of the reproducing region RD or DT in the erasable type orrelief type reproducing method may be maintained by controlling thelaser power to assure stable reproduction at all times.

Meanwhile, the circuit for generating the reproducing power settingreference values REF may be constituted by using, instead of ROM 24, acircuit adapted for finding the reproducing laser power settingreference value REF by processing the track address information.

Instead of changing the laser power setting reference values for therespective tracks, each one laser power setting reference value may beassociated with each set of plural tracks. In this case, a laser powersetting reference value associated with a linear velocity at a centraltrack of the plural tracks may be used as a laser power settingreference value for the track set.

Although the laser power is controlled in the above embodiment forrendering the size of the reproducing regions RD and DT constant despitechanges in the temperature of the magneto-optical disc, similar effectsmay also be achieved by controlling the external magnetic field(reproducing magnetic field H_(read)).

That is, with the erasable type reproducing method, for example, thetemperature at which the mask region (recording erased region) ER startsto be generated is precisely not the Curie temperature T_(c2) of theintermediate layer 62, but is correlated with the reproducing magneticfield H_(read), and is a temperature such that

    H.sub.c1 +H.sub.W <H.sub.read                              (1)

wherein H_(c1) is coercivity of the reproducing layer 61 and H_(W) isthe exchange-coupling force between layers 61 and 63. Theexchange-coupling force H_(W) between the reproducing layer 61 and therecording layer 63 is decreased with a rise in temperature and becomesequal to zero at the Curie temperature T_(c2) of the intermediate layer62.

FIG. 8 shows temperature characteristics of H_(cl) +H_(W). In FIG. 7,T_(c1) is the Curie temperature of the reproducing layer 61. At atemperature higher than the Curie temperature T_(c2) of the intermediatelayer, coercivity is similar to that for a sole reproducing layer.

For aligning the domains of the reproducing layer 61 of themagneto-optical disc, it suffices to apply a magnetic field larger thanH_(c1) +H_(W), as shown by the formula (1). Therefore, if H_(r0) isapplied as reproducing magnetic field H_(read) in FIG. 8 for the sametemperature distribution, the region at a temperature higher than theCurie temperature T_(c2) becomes the mask region ER. However, if thestrength of the reproducing magnetic field H_(read) is H_(rl), the rangeup to a temperature T_(a) lower than the Curie temperature T_(c2)becomes the mask region ER. In this manner, the size of the mask regionis changed with the strength of the reproducing magnetic field H_(read),so that the reproducing region RD is changed in size.

Therefore, by changing the external magnetic field, such as thereproducing magnetic field H_(read), depending on the temperature of themagneto-optical disc 11, the reproducing region may perpetually berendered constant.

The reproducing magnetic field may similarly be controlled in the caseof the relief type reproducing method for rendering the size of thereproducing region DT constant.

FIG. 9 shows an embodiment of essential parts of a reproducing apparatusin which the reproducing magnetic field is controlled depending on thelinear velocity of the magneto-optical disc. Similarly to the precedingembodiment, the magneto-optical disc 11 is control led with constantangular velocity (CAV) system.

In the present embodiment, the constant laser power setting referencevalue REF from a reference value generator 25 is supplied to theautomatic power controlling circuit 22 and an output laser power of thelaser light source 12 is controlled to a constant value conforming tothis reference value REF.

A reference value M_(ref) from reference value generator 33 is suppliedto an adder 34 where it is added to a correction value from a ROMadapted for generating correcting values 35. A driving signal consistingin the produced sum value is supplied to a driver 32. Thus the strengthof the reproducing magnetic field H_(read) is of a predetermined valueconforming to the reference value REF, if the correcting value is zero,so that the strength is changed around the predetermined value dependingon the correcting value.

In the present embodiment, ROM 35 stores a table of the correctingvalues conforming to the linear velocities of the magneto-optical disc11 at the reproducing positions. Track addresses from the addressdecoder 17 are entered as readout addresses of ROM 35. In the presentembodiment, the correcting values stored in ROM 35 are of such valuesthat the sizes of the reproducing regions RD and DT become perpetuallyconstant for respective linear velocities of the magneto-optical disc 11which are different with different radial reproducing positions of themagneto-optical disc 11.

Whether or not the size of the reproducing region RD and Dt is constantmay again be detected depending on whether or not the RF signal levelfrom signal processor 15 is of a constant value when the information ofa predetermined reference pattern is reproduced.

In this manner, track addresses are detected from playback signals bythe address decoder 17 during reproduction. Based on these trackaddresses, the correcting values conforming to the linear velocities atthe reproducing positions are read out from ROM 35 so as to be suppliedto an adder 34. This controls the strength of the reproducing magneticfield H_(read) to render the size of the reproducing region RD or DTconstant at all times.

Meanwhile, a circuit for calculating the correcting values from theinformation concerning the detection temperature from the temperaturesensor may be used as the correcting value generator in place of ROM 35.

As in the above mentioned embodiment the laser power and the externalmagnetic field may also be controlled simultaneously, in place ofindependently controlling the laser power and the external magneticfield depending on linear velocities of the magneto-optical disc at thereproducing positions.

Although the reproducing positions, that is radial positions of theoptical pickup during reproduction on the magneto-optical disc 11, aredetected in the above-described embodiment by extracting track addressesin the playback signals, the optical pickup position may also bedetected by a position sensor.

In an embodiment for the latter case, shown in FIG. 10, both the laserpower and the reproducing magnetic field are controlled simultaneously.

In this figure, an optical pickup 40 is provided with a laser lightsource 12, photodetectors 13 and 21 and an optical system, not shown,and is adapted for being slid along the radius of the magneto-opticaldisc 11 by a radial translation unit 41.

A position sensor 40, such as a potentiometer, is provided in the radialfeed unit 41. The position of the laser beam scanning spot from theoptical pickup 42 along the radius of the magneto-optical disc 11, thatis the reproducing position, may be detected by the position sensor 42.An output of the position sensor 42 is supplied to a reproducingposition discriminating circuit 43. An output from the reproducingposition discriminating circuit 43, indicating the reproducing positionon the magneto-optical disc 11, is supplied as readout addresses to theROM 24 adapted for producing the reproducing laser power settingreference values and to the ROM 35 adapted for producing correctingvalues for correcting the reproducing magnetic field H_(read).

In the present embodiment, ROMs 24 and 35 store sets of the laser powersetting reference values REF and the correcting values which areassociated with different linear velocities and which will give constantsizes of the reproducing region RD or DT even although the linearvelocities at the respective reproducing positions are changed.

The laser power and the reproducing magnetic field may be controlled inthis manner depending on the linear velocities at the reproducingpositions, such that, similarly to the above-described embodiment,reproduction may be achieved by the light reflected from the reproducingregion RD or DT of the constant size at any reproducing position of themagneto-optical disc 11 to assure stable reproduction at all times.

In the present embodiment, the radial extent of the magneto-optical disc11 may also be divided into plural ranges and a set of the laser powersetting reference values REF and the correcting values may be associatedin a one-for-one relationship with a representative linear velocity foreach range so that a different value of the set may be read from ROMs 24and 35 for each range.

Similarly to the preceding embodiment, a processing circuit for findingthe reproducing laser power setting reference value REF and thecorrecting value from the information concerning the radial positionfrom the position sensor 42 may also be employed in place of the ROMs 24and 35.

Although the magneto-optical disc is rotationally driven by a constantangular velocity (CAV) system, the present invention may also be appliedto a so-called modified CAV rotational driving system.

That is, although the CAV system is employed in the magneto-optical discof the modified CAV system, the radial extent of the disc is dividedinto plural zones ZN, as shown in FIG. 11, and recording/reproduction isperformed using data clock frequencies which are varied from one zone toanother in such a manner as to render the linear recording density ineach zone ZN from the inner periphery towards the outer peripherysubstantially constant to increase the recording density.

Although the linear recording density of the recording pits RP ischanged in each zone ZN, changes in the linear recording velocities maybe substantially disregarded with respect to the clock frequencies.

Meanwhile, the recording density may be increased by adopting theconstant linear velocity (CLV) system as the rotating driving system.However, since it is necessary with CLV to change the number ofrevolutions depending on the track positions of the magneto-opticaldisc, control of the number of revolutions of the spindle motor iscomplicated during data accessing to lower the accessing speed. With themodified CAV system, the rotational driving may be achieved by CAV toraise the accessing speed to improve the recording density to achievehigh speed data accessing.

However, since the magneto-optical disc is rotated at a constant numberof revolutions, the linear velocities become different depending on thereproducing positions along the disc radius and hence the sizes of theeffective reproducing region RD or DT become different, as in thepreceding embodiment.

In this consideration, with the reproducing apparatus for themagneto-optical disc of the modified CAV rotating and driving system,the laser power and/or the reproducing magnetic field is controlled inaccordance with the linear velocity at the reproducing position torender the size of the reproducing region constant to effect stabilizedreproduction. With the present modified CAV system, since theinformation concerning which of the zones counted from e.g. the innerperiphery of the disc is the current zone is written in data, the radialposition of the current zone may be detected from the zone informationand a linear velocity may be presupposed from one zone to another on thebasis of the zone information for performing the above-mentioned controlof the laser power or the reproducing magnetic field.

It is seen from above that, by combining the modified CAV rotating anddriving system with the erasable or relief type reproducing methods,recording/reproduction may be achieved with a higher density than thatachieved by employing the CAV rotating and driving system. Besides, ahigher accessing speed than that achieved with CLV may be achieved byapplying the reproducing method of the present invention to amagneto-optical disc for data recording.

The present invention may also be applied to a magneto-optical disc ofthe type consisting in a mixture of the erasable and relief types.

With the high density reproducing technology employing thesemagneto-optical recording media, recording pits may be read only fromreproducing regions narrower than the beam spot area. Besides, theeffective size of the reproducing region may perpetually be renderedconstant despite changes in the linear velocities of the recording mediato provide for stable reproduction. The result is that high density maybe achieved to increase the capacity of the recording medium to producehigh quality reproducing signals at all times.

The above embodiments are directed to a recordable magneto-opticalrecording medium. The following description is made of an embodiment inwhich the present invention is applied to a variable reflectancemagneto-optical recording medium.

As the technique concerning the variable reflectance optical recordingmedium, the present Applicant has already proposed a signal recordingmethod for an optical disc in the specification and drawings of JapanesePatent Application No. H-2-94452 (1990), and an optical disc in thespecification and drawings of Japanese Patent Application NO. H-2-291773(1990). In the former, a signal reproducing method for an optical discis disclosed, whereby a readout light is radiated to an optical disc onwhich phase pits are formed depending on signals and which hasreflectance values changed with temperatures, and the phase pits areread while the reflectance is partially changed within a scanning lightspot of the readout beam. In the latter, an optical disc of a so-calledphase change type is disclosed, in which a layer of a material changedin reflectance with phase changes is formed on a transparent substratewhich has reflectance values changed with phase changes and in which,when the disc is irradiated with the readout beam, the layer partiallyundergoes phase changes within the scanning spot of the readout beam andis reset after readout is terminated.

The material of the layer is preferably such a material in which a layerof a phase change material which may be crystallized after being meltedand in which, when the layer is irradiated with the readout beam, thematerial is changed into a liquid phase within the scanning spot of thereadout beam within the melted and crystallized region so as to bechanged in reflectance and is reset to a crystallized state afterreadout is terminated.

FIG. 12 shows essential parts of a disc reproducing apparatus to whichis applied a modified embodiment of the reproducing method of thepresent invention employing the variable reflectance type opticalrecording medium, above all, the phase change type optical disc.

In FIG. 12, an optical disc 100 is a variable reflectance type opticaldisc, above all, a phase change type optical disc. The disc in which thereflectance of a portion thereof irradiated with the readout laser beamand raised in temperature is lower than that of the remaining portioncorresponds to the erasable type magneto-optical disc, while the disc inwhich the reflectance of a portion thereof raised in temperature ishigher than that of the remaining portion corresponds to the relief typemagneto-optical disc. The present embodiment is applicable not only toboth types of the phase change type optical discs, but also to variablereflectance type optical discs based on another operating principle.

The arrangement shown in FIG. 12 is the same as that shown in FIG. 5except that the magnetic field generating coil 21, driver 22 and thereference value generator 33 are eliminated and a variable reflectancetype optical disc 100 is used in place of the magneto-optical disc 11.

That is, in the present embodiment, the optical pickup position of thereflectance type optical disc 100, that is the reproducing position, isdetected by detecting the above-mentioned track address, and the laserbeam power is controlled in accordance with the linear velocity at thereproducing position to control the size of the portion within the beamspot of the laser beam where reflectance is changed to maintain theconstant size of the high reflectance portion at all times within thebeam spot which is the effective reproducing region.

That is, a light beam from the laser light source 12 is incident on theoptical disc 100 and the beam reflected from a reproducing region withinthe laser beam spot is incident on a reproducing photodetector 13 toundergo photoelectric conversion while output signals from photodetector13 are supplied by means of head amplifier 14 to signal processor 15 toproduce RF signals which are supplied to a data reproducing system fordemodulation.

Part of the laser beam from the laser light source 12 is incident on aphotodetector 16 for laser power monitoring to undergo photoelectricconversion before being supplied to the automatic power controller 17.In the automatic controller 17, an output of the photodetector 16 andthe reproducing laser power setting reference value REF are compared toeach other. A comparison error output from controller 17 is supplied tolaser driving circuit 23 for controlling the output power of the laserlight source 12.

Part of the laser beam from the laser light source 12 is incident on thelaser power monitor photodetector 21. By means of the above-mentionedclosed loop control, the photoelectrically converted output ofphotodetector 21 is controlled so that the output power of the laserlight source 12 is equal to a value conforming to the reproducing laserpower setting reference value REF. The setting reference value REF isset so as to conform to the linear velocity at each reproducing positionalong the radius of the variable reflectance optical disc 100.

To this end, a ROM 24 is provided for storing a table of reproducinglaser power setting reference values REF associated in a one-for-onerelation to the linear velocities at the track positions of the opticaldisc 100. In this case, such reproducing laser power setting referencevalues REF which will give at all times constant sizes of the effectivereproducing region of the optical disc 100 most proper for reproductionfor the states of the linear velocities for the respective reproducingtrack positions are previously detected and stored in the ROM 24.Whether the size of the reproducing region is of an optimum constantsize may be detected depending on whether or not the RF signal levelfrom signal processor 15 is of a predetermined value when theinformation of e.g. a predetermined reference pattern is reproduced.

In the address decoder 17 of the data reproducing system 16, trackaddresses are extracted from playback signals and discriminated. Thesetrack addresses are supplied as readout addresses to the ROM 24.Different values of the reproducing laser power setting values REF areread out from the ROM 24 depending on different linear velocities at thereproducing track positions. The read-out setting reference values REFare supplied to an automatic power controller 22 for controlling theoutput power of the laser light source 12 so as to conform to thesetting reference value REF in accordance with the linear velocity atthe current reproducing position on the optical disc 100.

In the case of the variable reflectance type optical disc 100, as in theabove-described magneto-optical disc, if the reproducing position alongthe radius of the optical disc 100 is changed the temperaturedistribution for the laser beam scanning spot is changed with the linearvelocity of the disc at the current reproducing position. By controllingthe laser power in the above-described manner, the size of thereproducing region may be maintained constant although the linearvelocity at the reproducing position of the magneto-optical disc 11 ischanged. It will be seen from above that, even if the reproducingposition along the radius of the variable reflectance type optical disc100 and hence the linear velocity is changed, the reproducing region maybe maintained at a constant size by controlling the laser power, so thatstable reproduction may be achieved at all times.

The embodiment shown in FIG. 12 may be modified in the same manner aswhen employing the magneto-optical disc. For example, the readout beammay be controlled in intensity on the basis of the linear velocity withdisc rotation, or the size of the portion within the beam spot in whichreflectance is changed on the basis of the level of the signal read outfrom the optical recording medium. The setting values may also be foundby processing instead of by using the ROM 24. The present invention mayalso be applied to the case of using the above-mentioned modified CAVrotating and driving system.

As an example of the optical disc 100 of the variable reflectance typeemployed in the embodiment of FIG. 12, a phase change type disc isexplained, in which a layer of a phase change material which may becrystallized after melting is used and in which, when the layer of thephase change material is irradiated with the readout beam, the layer ispartially liquefied in a melted and crystallized region within thereadout beam spot so as to be changed in reflectance, with the thusliquefied layer being reverted to the crystal state after readout isterminated.

Referring to a schematic cross-sectional view of FIG. 13, showing thephase change type optical disc, used as the optical disc 100 shown inFIG. 12, a layer of a phase change material 104 is formed via a firstdielectric layer 103 on a transparent substrate 102 on which phase pitsare formed (on the lower side in the drawing), a second dielectric layer105 is formed on the layer 104 (on the lower side in the drawing,hereinafter the same) and a reflecting layer 106 is formed on the seconddielectric layer. Optical characteristics, such as reflectance, are setby these first and second dielectric layers 103 and 105.

If necessary, a protective layer, not shown, may be additionallydeposited on the reflecting layer 106.

As alternative constitutions of the phase change type optical discs,only the phase change material 104 may be intimately deposited directlyon the transparent substrate 102 on which pits are formed, as shown inFIG. 14, or a first dielectric layer 103, a phase change material layer104 and a second dielectric layer 105 may be sequentially formed on thetransparent substrate 102 on which phase pits are formed, as shown inFIG. 15.

The transparent substrate 102 may be a substrate of synthetic resin,such as polycarbonate or methacrylate, or a glass substrate.Alternatively, a photopolymer layer may be deposited on the substrateand phase pits 101 may be formed by a stamper.

The phase change material may be such material which undergoes partialphase changes within a scanning spot of the readout beam and is resetafter readout and the reflectance of which is changed with phasechanges. Examples of the material include calcogenites, such as Sb₂ Se₃,Sb₂ Te₃, that is calcogen compounds, other calcogenites or unitarycalcogenites, that is calcogenitic materials, such as Se or Te,calcogenites thereof, such as BiTe, BiSe, In--Se, In--Sb--Te, In--SbSe,In--Se--Tl, Ge--Te--Sb or Ge--Te. If the phase change material phase 104is constituted by calcogen or calcogenite, its characteristics, such asheat conductivity or specific heat, may be rendered desirable forproviding a satisfactory temperature distribution by the laser readoutbeam. Besides, the melted state in the melted and crystallized region aslater explained may be established satisfactorily to generate ultra-highresolution with high S/N or C/N ratio.

The first dielectric layer 103 and the second dielectric layer 105 maybe formed of, for example, Si₃ N₄, SiO, SiO₂, AlN, Al₂ O₃, ZnS or MgF₂.The reflective layer 106 may be formed of Al, Cu, Ag or Au, admixed withminor amounts of additives, if desired.

As a concrete example of the phase change type optical disc, an opticaldisc having an arrangement shown in FIG. 13 is explained. With thisoptical disc, a layer of a material which may be crystallized afterbeing melted is formed on a transparent substrate on which phase pitsare previously formed. When a readout beam is radiated, the layer of thephase change material is partially liquefied in a melted andcrystallized region within the readout scanning light spot and is resetto the crystallized state after readout of a crystallized state.

A so-called glass 2P substrate was used as the transparent substrate 102of FIG. 13. Phase pits 101 formed on a major surface of the substrate102 were of a track pitch of 1.6 μm, a pit depth of about 1200 Å and apit width of 0.5 μm. A first dielectric layer 103 of AlN was depositedon one major surface of the transparent substrate 102 having these pits101, to a thickness of 900 Å, and a layer of a phase change material 104of Sb₂ Se₃ was deposited on the layer 103 (on the lower surface thereofin FIG. 13, hereinafter the same). A second dielectric layer 105 of AlNwas deposited thereon and an Al reflective layer 106 was depositedthereon to a thickness of 300 Å.

The following operation was performed on a portion of the optical discfree from recorded signals, that is a mirror-surface part thereof freefrom phase pits 101.

A laser beam of e.g. 780 nm was radiated to be focused on a point of theoptical disc, which was then initialized by being allowed to coolgradually. The same point was then irradiated with a laser pulse with alaser power P set to 0<P≦10 mW. The pulse width was set to 260nsec≦t≦2.6 μsec. The result is that, if the reflectance is changedbetween two solid phase states before pulse irradiation and after pulseirradiation followed by cooling to room temperature, the layer ischanged from a crystal state to an amorphous state. If the reflectanceis not changed during this operation, but the amount of return light isonce changed during irradiation of the pulse light, it is an indicationthat the film of the crystal state is once liquefied andre-crystallized. The region in the melted state which has once becomeliquid and which may be returned to the crystallized state with loweringof temperature is termed a melted and crystallized region.

FIG. 16 shows the phase states of the layer of the phase change material104 of Sb₂ Se₃ and values of a pulse width of the radiated laser pulseand the laser beam power plotted on the abscissa and on the ordinate,respectively.. In this figure, a hatched area R₁ below a curve aindicates a region in which the layer of the phase change material 104is not melted, that is maintained in its initial state. In this figure,the region above curve a becomes liquid, that is melted, on laser spotirradiation. A region between curves a and b is a melted andcrystallized region which is reset to a crystal state when cooled toabout the ambient temperature by elimination of the laser beam spot andthereby turned into a solid phase. Conversely, a hatched region R₃ abovecurve b is a melted amorphous region which is rendered amorphous whencooled and turned into a solid phase by elimination of the laser beamspot.

In the present embodiment, the reproducing laser power, optical discconstitution, material type and the film thicknesses are selected sothat, in the course of cooling to ambient temperature from the heatedstate caused by readout beam radiation during reproduction, the time Δtwhich elapses since the heated state brought about by radiation of thereadout beam during reproduction until cooling to ambient temperaturebecomes longer than the time necessary for crystallization, so that thestate of liquid phase in the melted and crystallized region R₂ in FIG.16 will be produced during reproduction.

In the present embodiment, the reflectance in the initial state, that isin the crystallized state, was 57%, whereas that in the melted state was16%. When reproduction was performed with the playback power of 9 mW andthe linear velocity of 3 m/sec, the ratio C/N was 25 dB.

FIG. 17 shows the results of measurement of the phase change states foranother example of the phase change type optical disc making use of Sb₂Te₃ as a phase change material 104, similarly to FIG. 16. In FIG. 17,the parts corresponding to those of FIG. 16 are indicated by the samereference numerals. In the present example, making use of Sb₂ Te₃, thereflectance in the crystallized state, that is initial state, was 20%,while that in the melted state was 10%.

Meanwhile, with calcogenites or chalcogens, such as Sb₂ Se₃ or Sb₂ Te₃,the reflectance for the amorphous state is substantially equal to thatin the melted state. The optical disc employed in the present embodimentis reproduced with an ultra-high resolution by taking advantage oftemperature distribution within the scanning spot on the optical disc.

Referring to FIGS. 18A and 18B, explanation is given of the case i whicha laser beam is radiated on the phase change type optical disc.

In FIG. 18B, the abscissa indicates a position of the light spotrelative to the scanning direction X. A beam spot SP formed on theoptical disc on laser beam radiation has a light intensity distributionas indicated by a broken line a. On the other hand, temperaturedistribution in the layer of the phase change type material 104 isshifted slightly rearward relative to the beam scanning direction X, asindicated by a solid line b, in association with the scanning speed ofthe beam spot SP.

If the laser beam is swept in the direction shown by arrow X, theoptical disc as a medium is gradually raised in temperature, from thedistal side relative to the scanning direction of the light beam spotSP, until finally the temperature becomes higher than the melting pointMP of the layer 104. In this stage, the layer 104 is in the meltedstate, from its initial crystal state, and is lowered in e.g.reflectance, as a result of transition to the melted state. Thereflectance of a hatched region P_(X) within the beam spot SP islowered. That is, the region P_(X) in which the phase pit 101 can hardlybe read and a region P_(Z) remaining in the crystallized state existwithin the beam spot SP. That is, even when two phase pits 101, forexample, exist in one and the same spot SP as shown, it is only thephase pit 101 present in the high reflectance region P_(Z) that can beread, whereas the other phase pit 101 is present in the region P_(X)with extremely low reflectance and hence cannot be read. In this manner,only the single phase pit 101 can be read even although plural phasepits 101 exist in the same spot SP.

Therefore, if the wavelength of the readout beam is λ and the numericalaperture of the objective lens is NA, readout can evidently be madesatisfactorily even with the minimum phase pit interval of the recordingsignals along the scanning direction of the readout beam of not morethan λ/2NA, so that signals can be read with ultra-high resolution toincrease the recording density, above all, the line density, and hencethe recording capacity of the recording medium.

In the above embodiment, operating conditions, such as film thicknesses,are set so that the reflectance is low or high when the layer of thephase change material 104 is in the melted state or in the crystallizedstate, respectively. However, the thickness or the constitution of eachlayer or the phase change material may be so set that the reflectancebecomes high or low in the melted state or in the crystallized state,respectively, in which case a phase pit 101 may be present in the hightemperature region P_(X) in the laser beam spot SP shown in FIG. 18A sothat only this phase pit in the high temperature region P_(X) is read.In the case of an irreversible phase change in which a region is rasedin temperature by laser light irradiation to reach the melted andcrystallized region R₃ such that it cannot be reset to the initializedstate or crystallized state even if cooled to ambient temperature, it isonly necessary to perform some initializing operation within the scopeof the present invention. For example, by radiating an elliptical spotafter the reproducing laser spot for heating the layer 104 to the meltedand crystallized region R₂, or by heating to a temperature lower thanthe melting point MP and not lower than the crystallization temperature,the layer 104 may be initialized by being reset from the amorphous stateto the crystallized state.

Although the reflectance is changed in the above embodiment by phasechanges of the recording medium, the reflectance may be changed bytaking advantage of any other phenomenon. Thus, for example, thereflectance may be changed by temperature by taking advantage of changesin spectral characteristics caused by moisture adsorption by aninterference filter according to a modified embodiment shown in FIG. 19.

In the embodiment shown in FIG. 19, materials with markedly differentrefractive indices are repeatedly deposited on a transparent substrate132, on which phase pits 131 are previously formed, to thicknesses equalto one fourths of the wavelength λ of the reproducing beam, for formingan interference filter. In the present embodiment, an MgF layer 133(with a refractive index of 1.38 ) and an ZnS layer 134 (with arefractive index of 2.35) are used as the materials with markedlydifferent refractive indices. However, any other combinations of thematerials having larger differences in refractive indices may beemployed. For example, SiO having a lower refractive index of 1.6 may beused as a low refractive index material, and TiO₂ with a refractiveindex of 2.73 or CeO₂ with refractive index of 2.35 may be used as ahigh refractive index material.

The above-mentioned MgF layer 133 or the ZnS layer 134 are deposited byevaporation. If the reached vacuum is set to a value of e.g. 10⁻⁴ Torrwhich is lower than a usual value, the film structure becomes porous topermit the moisture to be captured. With the interference filter formedby a film which thus has captured the moisture, the reflectance andspectral characteristics are changed markedly between the state in whichthe filter is at room temperature and the state in which the filter isheated to close to the boiling point of water, as shown in FIG. 20. Thatis, an acute wavelength shift is observed, in which the spectralcharacteristics at room temperature are as shown by a curve i having apoint of inflection at a wavelength λ_(R) while the characteristics atapproximately the boiling point are as shown by a curve ii having apoint of inflection at wavelength λ_(H) and returned to thecharacteristics shown by curve i on lowering the temperature. Thisphenomenon may be probably caused by acute changes in refractive indexdue to vaporization of the moisture resulting in changes in spectralcharacteristics.

Therefore, if the wavelength of the light source of the reproducing beamis selected to a wavelength λ₀ intermediate between these points ofinflection λ_(R) and λ_(H), the reflectance is dynamically changedbetween the state of room temperature and the heated state.

In the present embodiment, high density reproduction is performed bytaking advantage of these changes in reflectance. The mechanism of highdensity reproduction is described in connection with FIG. 18. In thiscase, the region in which the moisture is vaporized to producewavelength shift corresponds to the high reflectance region, while theportion of the medium in which the temperature is not raised is the maskregion. In the present embodiment, the reflectance characteristics arereverted to the original state when the temperature is lowered, so thatno particular erasure operation is required.

By using the reflectance change type optical disc as the optical disc100 shown in FIG. 12, the size of the effective reproducing region (thehigher reflectance region of the regions P_(X) and P_(Z) in FIG. 18) maybe rendered constant despite temperature changes of the optical disc100, so that reproduction may be performed stably to assure high qualityreproduction signals.

It is to be noted that the present invention is not limited to theabove-described embodiments, but may be applied to, for example, a card-or sheet-shaped optical recording medium besides the disc-shapedrecording medium.

We claim:
 1. A method for reproducing information on an opticalrecording medium comprising a recording layer and a reproducing layer,said recording and reproducing layers being magnetically coupled to eachother in steady state, said method comprising:extinguishing magneticcoupling between the recording layer and the reproducing layer in aregion the temperature of which is raised to a temperature higher than apredetermined temperature by irradiation of a readout laser beam duringreproduction, reading the recording information held by said recordinglayer in an area of an irradiated region other than the magneticcoupling extinguished region, detecting a reproducing position on saidoptical recording medium when rotating said optical recording medium ata constant rotational velocity for reproducing said recording medium,and controlling the size of the magnetic coupling extinguished region inaccordance with changes in the linear velocity at said reproducingposition.
 2. The method for reproducing information on an opticalrecording medium comprising a recording layer and a reproducing layer,said recording and reproducing layers being magnetically coupled to eachother in steady state, said method comprising:extinguishing magneticcoupling between the recording layer and the reproducing layer in aregion the temperature of which is raised to a temperature higher than apredetermined temperature by irradiation of a readout laser beam duringreproduction, reading the recording information held by said recordinglayer in an area of an irradiated region other than the magneticcoupling extinguished region, detecting a reproducing position on saidoptical recording medium when rotating said optical recording medium ata constant rotational velocity for reproducing said recording medium,controlling the size of the magnetic coupling extinguished region inaccordance with the linear velocity at said reproducing position, andcontrolling an output of a laser light source radiating the readout beamto said optical recording medium based on an output detecting thereproducing position of said optical recording medium.
 3. The method asdefined in claim 2, further comprising:controlling the output of thelaser light source based on a comparison output between the outputdetecting the reproducing position of said optical recording medium andan output reference value of the laser light source associated with thelinear velocity of the optical recording medium.
 4. The method asdefined in claim 2, further comprising:controlling the size of a secondregion based on the level of the output reproducing said opticalrecording medium.
 5. A method for reproducing information on an opticalrecording medium having a recording layer and a reproducing layer, saidmethod comprising:aligning domains of the reproducing layer,transcribing the recording information to said reproducing layer andrelieving the transcribed information, said recording information beingheld before transcription by a region on said recording layer thetemperature of which is increased by irradiation with a readout beamduring reproduction, reading said recording information from a relivedregion of said reproducing layer, detecting a reproducing position onsaid optical recording medium when rotating said optical recordingmedium at a constant rotational velocity for reproduction, andcontrolling the size of said relieved region in accordance with changesin the linear velocity at said reproducing position.
 6. The method asdefined in claim 5, further comprising:irradiating said readout beam ofa laser light source based on an output detecting the reproducingposition on said optical recording medium.
 7. The method as defined inclaim 6, further comprising:controlling an output of said laser lightsource based on a comparison output between the output detecting thereproducing position on said optical recording medium and an outputreference value of said laser light source associated with the linearvelocity of said recording medium stored in storage means.
 8. The methodas defined in claim 5, further comprising:controlling the strength of anexternal magnetic field impressed on said optical recording medium byexternal magnetic field generating means from the opposite side of saidoptical recording medium relative to said readout beam.
 9. The method asdefined in claim 5, further comprising:controlling the strength of anexternal magnetic field from an external magnetic field generating meansby adding the output detecting the reproducing position of said opticalrecording medium and a correcting value associated with the linearvelocity of the optical recording medium stored in storage means to aninput signal of said external magnetic field generating means positionedto impress the external magnetic field on said optical recording mediumfrom the opposite side of said readout beam with respect to said opticalrecording medium.
 10. The method as defined in claim 5, furthercomprising:controlling the size of a magnetic coupling extinguishedregion based on the level of the output reproducing said opticalrecording medium.
 11. A method for reproducing information on an opticalrecording medium, comprising:radiating a readout beam to an optical discon which phase pits are formed in accordance with signals, the phasepits being changed in reflectance with temperatures, reading said phasepits with said readout beam and partially changing the reflectance ofsaid phase pits within a scanning spot of said readout beam as a resultof being impinged by said readout beam, detecting a reproducing positionon said optical recording medium when rotating said optical recordingmedium at a constant rotational velocity for reproduction, andcontrolling the size of a portion within the scanning spot of thereadout beam in which the reflectance is changed as the result of beingimpinged by said readout beam.
 12. The method as defined in claim 11,further comprising:controlling an output of a laser light sourceradiating said readout beam to said optical recording medium on thebasis of an output detecting the reproducing position of said opticalrecording medium.
 13. The method as defined in claim 12, furthercomprising:controlling the output of said laser light source on thebasis of a comparison output between the output detecting thereproducing position of said optical recording medium and an outputreference value of said laser light source which is stored in storagemeans and which is associated with the linear velocity of said opticalrecording medium.
 14. The method as defined in claim 11, furthercomprising:controlling the size of a portion of the scanning spot ofsaid readout beam which is changed in reflectance on the basis of thelevel of an output reproducing said optical recording medium.